There is a need for having liquid samples arranged in thin layers for their analysis. One of the reasons is that the useful propagation length within a sample of either an agent that modifies the sample before the analysis or a useful signal used in the analysis is often short. If the sample thickness is significantly bigger than this propagation length, the signal of interest can be distorted or diminished due to prolonged interactions with the sample or due to background noise pick up. For example, if a liquid sample is ablated with a laser and the sample thickness is significantly longer than the absorption length of the laser beam inside the sample, the analysis of the laser ablation plume can be hindered by the presence of the ablation recoil products that increase in amount with the sample thickness. These ablation recoil products are generated by impulsive momentum transfer from the primary laser ablation plume that expands rapidly shortly after the onset of laser ablation to the underlying sample. In case the used sample is soft (e.g. tissues or liquids), the recoil stress is typically strong enough to rapture the underlying sample and eject the fractured material into the secondary ablation plume consisting of recoil products. The recoil energy is not big enough for vaporization of the recoil products so these recoil products are a mixture of individual molecules, molecular clusters, and particulates of various sizes (typically 0.1-1 um range). In contrast to the composition of the primary plume which is well defined and function of the laser pulse energy, the mixture composition of the secondary plume is not. Since these two kinds of plumes occur close in time and space, it is not easy to separate their contribution to probe signals. This reduces the signal-to-noise ratio of acquired signals hence the phenomenon of secondary ablation plumes is not desirable when quantitative high-quality signals are looked for. A discussion of the drawbacks of the presence of laser ablation recoil products is found in the publication by: Kresimir Franjic and R. J. Dwayne Miller, “Vibrationally excited ultrafast thermodynamic phase transitions at the water/air interface”; Phys. Chem. Chem. Phys., 2010, 12, 5225-5239. Particularly, as shown in the time progression photographs of a water plume formed by a pulsed laser (FIG. 9 of this publication), the plume remains intact until about 1 microsecond after irradiation and then the plume disintegrates due to recoil. As noted above this is a typical result when the sample thickness is significantly longer than the absorption length of the laser beam inside the sample.
Another desirable feature of thin sample layers and the corresponding small layer volumes is reduced sample consumption. This feature can be substantial when analyzing samples that are expensive or available in small quantities, such as bioliquids.
Another desirable feature is simplicity and universality of sample preparation. Biological samples are often fragile, and many current sample preparation techniques involve complicated steps that are sample dependent. Such elaborate steps may introduce undesirable sample modifications and complicate interpretation of acquired data.